1. Field of the Invention
The invention relates to the use of expandable materials as a cement expansion additive. Polymer encapsulated organic chemicals are disclosed as examples of such expandable materials.
2. Background of the Invention
Cement is commonly used in residential, commercial, and industrial applications. While cement is largely chemically inert and has desirable strength characteristics, it has a very high density.
Various additives are used to reduce the density of cement, while maintaining desirable strength and durability of the set product. As cement sets, the volume decreases. Various additives can be used to increase the volume of cement, while maintaining desirable strength and durability of the set product. One such additive is aluminum powder. The addition of the powder to cement results in the formation of hydrogen gas. The gas causes the cement to expand. The reactivity of the aluminum powder is modified by coating it with a resin to delay the chemical reaction. Upon mixing with the cement and water, the chemical reaction typically starts to occur after about 45-60 minutes.
While hydrogen gas will provide some cement expansion, it does present a potential safety risk due to its flammability and potentially explosive nature. Additionally, the hydrogen gas bubbles generated are not encapsulated, there is a tendency for it to move in the cement matrix, and may rise to the surface and escape the cement, thereby eliminating any potential benefits to the cement. If the gas escapes before the cement slurry is set, then little or no benefits will be achieved.
Cementing is a common technique employed during many phases of wellbore operations. For example, cement may be employed to isolate or secure various casing strings and/or liners in a well (or primary cementing). In other cases, cementing may be used in remedial operations to repair casing and/or to achieve formation isolation or secondary cementing). In still other cases, cementing may be employed during well abandonment. Cement operations performed in wellbores under high stress conditions may present particular problems, among other things, difficulty in obtaining good wellbore isolation and/or maintaining mechanical integrity of the wellbore over the life of the well. These problems may be exacerbated in those cases where wellbore and/or formation conditions promote fluid intrusion into the wellbore, including intrusion of water, gas, or other fluids.
In a wellbore, cement may be used to serve several purposes. Among these purposes are to selectively isolate particular areas of a wellbore from other areas of the wellbore. For example, cement is commonly placed in the annulus created between the outside surface of a pipe string and the inside formation surface or wall of a wellbore in order to form a sheath to seal off fluid and/or solid production from formations penetrated by the wellbore. This isolation allows a wellbore to be selectively completed to allow production from, or injection into, one or more productive formations penetrated by the wellbore. In other cases cement may be used for purposes including, but not limited to, sealing off perforations, repairing casing leaks (including leaks from damaged areas of the casing), plugging back or sealing off the lower section of a wellbore, sealing the interior of a wellbore during abandonment operations, and so on.
The normal shrinkage of cement can pose problems during cementing in oil and gas wells. Shrinkage can play a major role in the long-term cemented annular isolation. The long term seal of the annulus is jeopardized if the set cement pulls away from the pipe, casing, or from the well bore. Formation of a “micro-annulus” can occur if the shrinkage is sufficiently severe. It is desirable to add one or more additives to expand the cement's initial volume, effectively offsetting the cement's shrinkage as it sets and hardens.
In using aluminum powder in oil and gas well applications to expand the cement, the produced hydrogen gas can also increase the chance of a well blowout, in addition to the safety issues discussed above.
Thus, there exists a need for cement expansion additives with attractive chemical and safety properties. These additives would preferably be easily handled, and would offset the shrinkage exhibited by cement as it sets.